1. Field of the Invention
The invention relates to a roller module, more particularly to a roller module for an automatic document feeder.
2. Description of the Related Art
U.S. Patent Application Publication No. 2006/0012104 A1 discloses an automatic document feeder as illustrated in FIG. 1. In use, a document sheet is inserted into the automatic document feeder by a first pick up roller 120 and is fed to a feed roller unit 130 through a dispensing roller 122 and a first roller unit 128. Afterward, the document sheet is conveyed from the feed roller unit 130 to a discharge roller unit 132 through an image scanning device 102, while a side of the document sheet is scanned. The document sheet can be reinserted into the automatic document feeder by a second pick up roller 140, and be fed to the feed roller unit 130 via an intermediate roller unit 142 for passing through the image scanning device 102 once again, such that an opposite side of the document sheet can be scanned. Since the feeding speeds of the first roller unit 128 and the intermediate roller unit 142 are both slightly faster than the feeding speed of the feed roller unit 130, tensile stress of the document sheet can be eliminated during the movement of the document sheet from either one of the first roller unit 128 and the intermediate roller unit 142 to the feed roller unit 130, such that skew of the document sheet can be corrected before the document sheet is advanced through the feed roller unit 130.
However, if the speed difference between the feed roller unit 130 and either one of the first roller unit 128 and the intermediate roller unit 142 is too large, the document sheet will deform seriously between the feed roller unit 130 and either one of the first roller unit 128 and the intermediate roller unit 142, which may result in a poor image scanning quality or even cause jamming of the document sheet. On the contrary, if the speed difference between either one of the first roller unit 128 and the intermediate roller unit 142 and the feed roller unit 130 is too small, the skew correcting effect of the automatic document feeder will be adversely affected.
U.S. Pat. No. 6,079,708 discloses a conventional roller module for an automatic document feeder as illustrated in FIG. 2. The conventional roller module includes a pair of parallel rollers 15, 16 that are adjacent to each other, and a skew correction film 17 that is made from an elastic material and that is disposed upstream of the rollers 15, 16. During movement of a document sheet 18 through the conventional roller module, a leading part of the document sheet 18 contacts and is slowed down by the skew correction film 17 before the document sheet 18 is fed through the rollers 15, 16. Since the remaining part of the document sheet 18 is still being fed toward the conventional roller module at that time, skew of the document sheet 18 can be corrected.
However, the skew correcting effect may vary according to qualities of the document sheet 18. Moreover, it is difficult to find the most suitable material for the skew correction film 17.